The present invention relates to an improved method of and apparatus for isolating liquid phase components from semisolid phase components of a biological fluid.
A conventionally employed technique for separating liquid phase components from semisolid phase components of a blood mixture utilizes centrifugation of the mixture. The serum or plasma is generally then isolated from the semisolid portion by pipette or decantation procedures. Because of the agitation to which the sample is subjected by the procedures, it often happens that some semisolid phase components enter the liquid phase components, and the method does not entirely separate all particulate matter from the desired serum or plasma. When the serum is to be subjected to diagnostic analysis with, for example, an automated electronic analyzer, it is important to employ a particulate-free specimen. Fibrin fibers are often the cause of analyzer malfunctions when they clog conduits or orifices in the analyzer.
To improve the quality of serum samples obtained using conventional procedures, blood component separation devices have been developed, such as those described in U.S. Pat. Nos. 3,481,477, 3,512,940 and 3,693,804. Although such devices isolate the desired blood serum or plasma from the semisolid portion of the sample, they are relatively expensive to construct, and as a result have not been as widely accepted commercially as the need suggests. In addition, blood component separation devices such as those described in the above patents, operate in part by exerting force or pressure upon the blood mixture to be separated. If too great a force is imposed upon the mixture, there is a danger that the serum or plasma will pass through the devices with sufficient velocity to force particulate matter into the serum portion. The isolation procedure with the devices is also usually carried out following separation of the blood components by centrifugation, which requires two distinct steps and consequent extension of time required for total serum preparation.
To avoid the increased time and expense required to use such blood component separation devices, as well as the potential for contamination of the serum portion with particulate matter, it would be desirable to provide some means for automatically obtaining a pure serum sample wherein, after centrifugation of the blood mixture, there exists between the liquid and semisolid phase components only a liquid-semisolid interface of serum and cell coagulum or a relatively inexpensive silicone separation barrier. Unfortunately, although the art provides various types of automated fluid samplers, none has been found suitable for the purpose.
One prior type of automated fluid sampler is described in U.S. Pat. No. 4,325,909, in which a vertically and horizontally movable arm structure has a fluid probe at its distal end for fluid sampling and transfer. The vertical position of the arm and probe are controlled by an electrical liquid level sensing circuit, such that upon movement of the probe toward and to the liquid sample, when the probe touches the surface of the sample a circuit is established to then further move the probe to a preselected fixed depth in the liquid. In consequence, since the vertical position of the liquid-semisolid interface of a blood mixture varies from sample to sample, the device is not entirely suitable for sampling the serum portion of a blood mixture, since for the purpose the probe should extend closely to but not beyond the interface, so that substantially all of the serum, but not particulate matter, is sampled.
Another automated apparatus for analyzing liquids is taught by U.S. Pat. No. 3,549,330, wherein a liquid sample is maintained in an airtight container while being moved from station to station and while necessary liquids are supplied to and removed from the container. However, the probe for removing and adding liquids is fixed for movement between a predetermined lowered position for removing or supplying liquid and a raised position for permitting rotation of a container tray and advancement of the next container to the operating station. Again, because the vertical position of the liquid-semisolid interface of a blood mixture varies from mixture to mixture, movement of the probe between fixed vertical positions makes this apparatus also less than entirely suitable for blood serum sampling, since there is no way to variably control proper positioning of the probe during sampling.